This invention relates to the recovery of a terephthalic acid from polyethylene terephthalate. In one aspect, this invention relates to recovery of terephthalic acid and ethylene glycol from waste products, such as beverage containers, made from polyethylene terephthalate.
Most polyester resins used in commercial applications are formed from raw materials which are rising in price and have relatively large markets. Accordingly, recovery of these raw materials from scrap, waste and used products is an important economical consideration as well as an ecological consideration. One widely used polyester is polyethylene terephthalate made from terephthalic acid and ethylene glycol.
Used bottles made from polyethylene terephthalate is a large potential source of recoverable terephthalic acid and ethylene glycol. It is estimated that from 375 to 500 million pounds of polyethylene terephthalate were used for beverage bottles in 1980. Recent legislation in several states requiring a deposit refundable upon return of all empty beverage containers has established an ongoing procedure for collecting and separating polyethylene terephthalate containers which must be disposed. Polyethylene terephthalate beverage containers cannot be reused because the elevated temperatures required for sterilization deforms the container.
Polyethylene terephthalate containers can be ground into small pieces for use as a filler material or remelted for formation of different articles. The polyethylene terephthalate recovered by such processes contains impurities, such as pigment, paper and metal from caps. Consequently, applications for the reclaimed polyethylene terephthalate are limited to non-food uses and low-purity molded products.
Several different techniques have been proposed for recovering terephthalic acid and ethylene glycol from polyethylene terephthalate. One technique involves depolymerization of polyethylene terephthalate by saponification.
In one approach for saponification, polyethylene terephthalate is reacted with an aliphatic alcohol and a dialkyl terephthalate is recovered. This approach is exemplified in U.S. Pat. Nos. 3,321,510, 3,403,115 and 3,501,420.
In a second approach, polyethylene terephthalate is reacted with an aqueous solution of an alkali metal hydroxide or carbonate (usually sodium hydroxide) at an elevated temperature to yield a water soluble salt of terephthalic acid and ethylene glycol. The reaction product is acidified to liberate terephthalic acid which is water insoluble and the terephthalic acid precipitate is separated by filtration or the like. This approach is exemplified by U.S. Pat. Nos. 3,377,519, 3,801,273 and 3,956,088. U.S. Pat. No. 3,544,622 discloses a variation to this approach wherein the reaction is carried out under conditions to produce a water insoluble salt of terephthalic acid which is separated, washed and then acidified to produce terephthalic acid.
Empty beverage containers recovered from the consumers commonly have aluminum caps lined with polyvinyl chloride or the like, wrap around polypropylene-coated paper labels bonded to the surface with a polyvinyl acetate adhesive, residual sugars and, in some cases, polyethylene base caps for strengthening purposes. Without costly controls, reaction conditions in the saponification processes disclosed in the above-noted patents tend to cause some dissolution of these extraneous materials which would become impurities in the recovered terephthalic acid and require costly purification. Therefore, various approaches have been considered for removing these materials from the containers prior to grinding or separating them from the polyethelene terephthalate after grinding. Such separation procedures represent a significant increase in the overall cost of recovery.